1. Field of the Invention
The present invention relates to an information recording medium capable of recording, reproducing and erasing information by irradiation with laser beam or the like.
2. Description of the Prior Art
The known types of optical disks include the write-once type disk using a thin film of TeOx (0&lt;.times.&lt;2.0) mainly composed of Te and TeO.sub.2. Besides, chalcogenide alloys (for example, Te.sub.85 Ge.sub.15, Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2) are known as the materials capable of recording information by heating the thin film with laser beam to melt it and cooling rapidly to amorphize it, and erasing by heating it and cooling slowly to crystallize. Developments of information recording medium capable of recording, reproducing and erasing information by using such materials are being promoted. For the purpose of increasing the thermal stability of the information recording medium, the stability of multiple cycles of recording and erasing, and optical absorption sensitivity or change of reflectivity by making use of the interference effect, protective layers excellent in heat resistance are disposed on both sides of the active layer. When recording information by laser beam on such recording medium, the active layer is preliminarily crystallized, and then irradiated with a laser beam which is intensity modulated corresponding to information and reduced to a beam diameter of about 1 .mu.m to be heated above the melting point of the active layer. After the heated part is cooled rapidly, the information is recorded as an amorphization mark. To erase the recorded information, the active layer is heated by a laser beam and is cooled slowly to crystallize again.
The problem in the information recording medium subjected to the cycle of heating to melt, rapid cooling and amorphizing, and the cycle of heating, slow cooling, and crystallizing is, above all, the variations of the signal quality due to the heating-cooling cycle. Causes of such variations include thermal and mechanical damages to the active layer due to quick repetition of heating and cooling over 400.degree. C. by laser beam irradiation. Besides, when the active layer is heated above the melting point, a chemical reaction occurs between the components in the active layer and the components in the protective layer being in contact therewith, which may cause deterioration of the active layer. As a preventive measure of such deterioration, for example, a protective layer composed of a mixture of chalcogenide compound such as ZnS and ZnSe and oxide such as SiO.sub.2 and GeO.sub.2 has been proposed in the Japanese Laid-open Patent No. 63-103453.
However, even when such protective layer is provided, in repetition of heating and cooling of over 1 million cycles, the components of the active layer and components of the protective layer react with each other to increase the noise and change the reflectivity, causing deterioration of the recording and erasing cycle characteristic in the order of 1 million cycles.